An existing device includes a housing with an opening which is closed by a lid, the lid having a frame hermetically sealed to a window, and the window being transmissive to radiation in a waveband of interest. The device can be used in a television or a projector to form images, which are typically projected onto some type of screen so that they can be viewed by a person. The device includes within the housing a digital micromirror device (DMD) of a known type. A beam of radiation enters the housing through the window in the lid, and is processed by the DMD to form a plurality of sub-beams which represent an image, at least some of the sub-beams then existing the housing through the window in order to facilitate generation of the image which is projected onto the screen.
One standard technique for making such a lid involves forming a metal frame with an opening through it, placing a glass window in the opening through the frame, and then heating the frame and window until the peripheral edges of the window became fused to the edges of the opening in the frame. While this approach has been adequate for its intended purposes, it includes some disadvantages. One disadvantage was that, in order to fuse the window to the frame, the window must be heated to a temperature above its melting point, which causes imperfections to be introduced into the glass material of the window. These imperfections have to be polished out after the window had been fused to the frame, and certain other steps therefore cannot be carried out until after the polishing has been completed, including the addition of anti-reflective (AR) coatings on each side of the window. The need to polish out imperfections and apply the AR coatings separately to each window involved manufacturing costs for each lid which were higher than desirable.
As an alternative, a subsequently developed technique used a different approach to secure the window to the frame. In particular, this alternative approach did not directly fuse the edges of the window to the frame, but instead used at least one ring of sealing glass to secure a side surface of the window to a side surface of the frame. By selecting the sealing glass to have a melting temperature lower than the melting temperature of the glass material of the window, the window did not have to be heated to its melting point, which in turn avoided the introduction of imperfections into the glass of the window while coupling the window to the frame.
Since this modified approach avoided introducing imperfections into the window, it also avoided the need for an additional polishing step after coupling the window to the frame. As a result, it permitted the AR coatings to be applied to both sides of a large sheet of glass before the sheet was cut into a number of separate windows, and thus before the windows were actually coupled to frames. This reduced the cost of apply AR coatings to the windows. However, even though this approach was adequate for its intended purposes, it was not satisfactory in all respects.
In particular, lids made by both of the foregoing techniques are subjected to a thermal shock test after being manufactured, in order to eliminate any lids that might not hold up over time in response to temperature variations encountered curing normal operational use. During the thermal shock test, each lid is alternately and repeatedly immersed in two liquids having respective temperatures of −55° C. and +125° C. As to lids made by fusing the edges of the window to the frame, a high percentage of the lids survive the thermal shock test without any cracking of the glass material forming the window. On the other hand, as to lids made by securing the window to the frame with a sealing glass, the percentage of lids surviving the thermal shock test is significantly lower, because these lids show a significantly higher tendency to develop cracks within the window during thermal shock test. This reduces the effective production yield, which in turn causes the unit cost for manufacturing each such lid to be higher than desirable. Therefore, even though use of a ring of sealing glass permitted lids to be made more cheaply than when directly fusing the window to the frame, the lids made with sealing glass still had a higher cost than would be the case if production yields could be increased through reduction of the occurrence of window cracks during the thermal shock test.